This invention relates to rotor blades and specifically to the mechanical damping of vibratory energy in the blades of rotor assemblies during operation.
Rotor assemblies are used in a variety of turbo-machines, such as turbines, compressors and the like. Regardless of the application, rotor assemblies generally include a disk mounted on a rotating shaft with a plurality of blades extending radially outward around the circumference of the disk. Rotor blades typically consist of a root portion in contact with the disk and an airfoil portion extending radially therefrom. Flow of the working fluid, i.e. air for jet engines, across the airfoil portion imparts a force upon the blades and results in rotation of the shaft. Blades may be manufactured as an integral component of the disk (blisk-type blades) or as separate components (mechanically attached blades). Mechanically attached blades are generally secured to the disk by the interlocking engagement of a dovetail root portion of the blade with a complementary dovetail slot in the disk, thereby preventing separation of the blade from the disk during operation.
During operation, fluid forces induce vibratory stresses on the blades, resulting in high cycle fatigue and potential failure of the blades. Dampers, commonly frictional dampers, are utilized to reduce the magnitude of these dynamic stresses, thereby increasing operational life of the blades.
Many conventional frictional damper designs are known. Blade-to-blade frictional dampers, located in the space between circumferentially adjacent blades, often near the blade root or the blade tip, are common. Blade-to-blade dampers are held in place by any of various known retaining means, such as a damper chamber or tip shroud. This prevents the damper from separating from its adjacent blades, but also allows some movement of the damper during operation for frictional damping. During operation, centrifugal forces urge the damper into frictional contact with its adjacent blades. This contact reduces the relative motion between the adjacent blades, thereby reducing the vibratory stresses on the blades during operation. Frictional damping is effective at any frequency so long as there exists relative motion between the damper and the blade. Exemplary blade-to-blade dampers are shown in U.S. Pat. No. 5,226,784 entitled xe2x80x9cBlade Damperxe2x80x9d issued to Mueller et al. on Jul. 13, 1993; and U.S. Pat. No. 4,722,668 entitled xe2x80x9cDevice for Damping Blade Vibrations in Turbo-Machinesxe2x80x9d issued to Novacek on Feb. 2, 1988, incorporated herein by reference.
Blade-to-ground frictional dampers are also well known in the art. Blade-to-ground dampers are placed between the turbine blade and an object secured to the disk. Frictional contact of the damper between the blade and the relatively stationary object similarly reduces the vibratory stresses on the blade. An exemplary blade-to-ground damper is shown in U.S. Pat. No. 5,052,890 entitled xe2x80x9cDevice for Damping Vibrations in Turbomachinery Bladesxe2x80x9d issued to Roberts on Oct. 1, 1991, incorporated herein by reference.
Also known in the art are internal dampers that are placed within a cavity inside the blade. These dampers similarly reduce vibratory stresses on the blade through frictional contact with an inner surface of the blade. Exemplary internal blade dampers are shown in U.S. Pat. No. 5,820,343 entitled xe2x80x9cAirfoil Vibration Damping Devicexe2x80x9d issued to Kraft et al. on Oct. 13, 1998; U.S. Pat. No. 5,407,321 entitled xe2x80x9cDamping Means for Hollow Stator Vane Airfoilsxe2x80x9d issued to Rimkunas et al. on Apr. 18, 1995; and U.S. Pat. No. 2,999,669 entitled xe2x80x9cDamping Apparatusxe2x80x9d issued to McGinnis on Sep. 12, 1961, all incorporated herein by reference.
Conventional frictional dampers suffer from several disadvantages. For example, dampers placed between blades are often difficult to install and remove. Also, many dampers placed between blades require the addition of a shroud or some other means to retain the damper during operation, which increases the weight of the blade. This increases the stresses experienced by the blade and can reduce operational life. Further, blade-to-blade and blade-to-ground frictional dampers are exposed to the harsh, corrosive environment of the working fluid. Damper corrosion compromises the effectiveness of the damper. Severe damper corrosion may allow the damper to break away from its retaining means, potentially damaging downstream components.
It is therefore desirable to develop a damper that is located outside the path of the working fluid that is easy to install and does not significantly increase the weight of the blade. Although internal dampers are located outside the path of the working fluid and most do not add significant weight to the blade, existing rotor blades, particularly blisk blades, may not be easily retrofitted with conventional internal damper designs. Also, installation and removal of conventional internal damper designs is difficult. Further, the relative motion between the blade and most internal dampers is generally minimal, limiting the effectiveness of the internal dampers.
In order to provide improved mechanical damping of rotor blade vibrations, the present invention provides a split-blade frictional damper that is positioned in a cavity defined by the body of the airfoil portion of the blade. The cavity has a generally radial elongated opening that extends entirely through the circumferential dimension of the airfoil and from the tip of the airfoil radially inward a specified distance, such as more than fifty percent of the height of the body. The elongated radial opening generally lies in a plane perpendicular to the axis of rotation of the rotor assembly but may be offset from this plane. This radial opening divides the airfoil portion of the blade into two distinct airfoil sections, each capable of motion relative to the other section. The cavity also may include an elongated opening transverse to and intersecting the radial opening, generally above the midpoint of the elongated radial opening. Typically, the elongated transverse opening lies in a plane parallel to the axis of rotation of the rotor assembly but may be offset from this plane. The transverse opening may, but need not, extend entirely through the circumferential dimension of the airfoil.
When the airfoil sections vibrate relative to one another, the frictional contact of the damper with the airfoil sections reduces this vibration and hence the stress on the blade as a whole. Thus, the portion of the damper in the elongated transverse opening has sufficient surface area to provide adequate frictional damping. The cross section of the damper is complementary with the cross section of the cavity thereby preventing passage of the working fluid through the cavity. Flow of the working fluid through the cavity reduces the flow across the surface of the blade and, therefore, the efficiency of the blade.